Convective outflow boundary initiates new convection over Kansas

July 31st, 2013 |
GOES-13 0.63 µm visible channel images (click image to play animation)

GOES-13 0.63 µm visible channel images (click image to play animation)

AWIPS images of 1-km resolution GOES-13 0.63 µm visible channel data (above; click image to play animation) showed an undular bore marking a surface-based convective outflow boundary which formed over southwestern Nebraska early in the day on 31 July 2013 — this outflow boundary then propagated southwestward during the day and acted as a focus for the formation of severe thunderstorms over southwestern Kansas later that afternoon (SPC storm reports).

The southwestward-propagating outflow boundary / undular bore feature was also seen on 4-km resolution GOES-13 6.5 µm water vapor channel imagery (below; click image to play animation).

GOES-13 6.5 µm water vapor channel images (click image to play animation)

GOES-13 6.5 µm water vapor channel images (click image to play animation)

The GOES-13 imager 6.5 µm water vapor channel weighting function calculated using the 12 UTC rawinsonde data from Dodge City, Kansas (below) indicated that the weighting function peaked much lower in the atmosphere (around 500 hpa) than normal — this allowed a thermal signal (albeit a faint one) of the boundary layer convective outflow boundary cloud features to be seen on the water vapor imagery.

GOES-13 water vapor channel weighting function plot (using Dodge City, Kansas rawinsonde data)

GOES-13 water vapor channel weighting function plot (using Dodge City, Kansas rawinsonde data)

A good view of the undular bore (which was trailing the leading edge of the convective outflow boundary) could be seen on a comparison of 1-km resolution MODIS 0.64 µm visible channel and 11.0 µm IR channel images at 17:05 UTC (below).

MODIS 0.64 µm visible channel and 11.0 µm IR channel images

MODIS 0.64 µm visible channel and 11.0 µm IR channel images

New thunderstorms formed along the old convective outflow boundary (as it encountered increasing instability across southwestern Kansas during the afternoon hours), as seen on 1-km resolution Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images (below). This storm was producing 1-inch diameter hail and wind gusts of 50-60 mph.

Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel mages, with SPC reports of hail

Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel mages, with SPC reports of hail

A comparison of the 1-km resolution Suomi NPP VIIRS 11.45 µm IR channel image with the corresponding 4-km resolution GOES-13 10.7 µm IR channel image (below) displayed an unusually large 30-degree difference between the coldest cloud-top IR brightness temperatures of the northernmost of the newly-formed thunderstorms in Kansas (-92º C on VIIRS, vs -62º C on GOES).

VIIRS 11.45 µm IR channel and GOES-13 10.7 µm IR channel images

VIIRS 11.45 µm IR channel and GOES-13 10.7 µm IR channel images

 

The Memphis Derecho of July 22 2003

July 31st, 2013 |
GOES-12 10.7 µm IR imagery (Click Image to play animation)

GOES-12 10.7 µm IR imagery (Click Image to play animation)

On the morning of July 22, 2003, a strong derecho moved through metropolitan Memphis, TN, with winds exceeding hurricane-force. The most significant impact of this storm was a loss of power caused in part by the many trees that were downed by the winds. The Storm Report for the day from the Storm Prediction Center shows a cluster of wind reports in and around Memphis and Shelby County. The National Weather Service office in Memphis produced a report on this event that includes radar imagery and a discussion of surface and upper-air observations. More information on this derecho is here. What do satellite data show for this event?

The animation of 10.7 µm imagery, above, shows the development of convection in southeast Kansas and northwest Arkansas that then moves eastward into the mid-South, hitting Memphis around 1200 UTC. Several overshooting tops are evident as the storms pass near Memphis, with the coldest brightness temperatures at 196K! Past derechosdiscussed on this blog (such as the one that hit the East Coast in 2012) were characterized by a channel of moisture and instability aligned with the storm motion, allowing the propagating thunderstorm complex access to a rich source of moisture and instability. This event in 2003 was no different. GOES-12 Sounder retrievals — during that year, 3×3 fields-of-view were used (versus single pixels now) — of Total Precipitable Water, Convective Available Potential Energy (CAPE) and Lifted Index (LI), show abundant moisture and instability aligned west-to-east across northern Arkansas. CAPE values exceeded 3000 J/kg, Total Precipitable Water was greater than 2 inches, and Lifted Indices were near -10.

GOES-10/GOES-12 Sounder-Derived Total Precipitable Water (3x3 Field of View) (Click Image to play animation)

GOES-10/GOES-12 Sounder-Derived Total Precipitable Water (3×3 Field of View) (Click Image to play animation)

GOES-10/GOES-12 Sounder-Derived Convective Available Potential Energy (CAPE) (3x3 Field of View) (Click Image to play animation)

GOES-10/GOES-12 Sounder-Derived Convective Available Potential Energy (CAPE) (3×3 Field of View) (Click Image to play animation)

GOES-10/GOES-12 Sounder-Derived Lifted Index (3x3 Field of View) (Click Image to play animation)

GOES-10/GOES-12 Sounder-Derived Lifted Index (3×3 Field of View) (Click Image to play animation)

GOES-12 Visible Imagery (Click Image to play animation)

GOES-12 Visible Imagery (Click Image to play animation)

Visible imagery, above, from GOES-12 shows the convection continuing to develop as it moves across the Mississippi River into Memphis. Several Overshooting tops are evident, as well as parallel cloud lines at the cirrus level that are usually associated with turbulence. GOES-10, as GOES-West, was also able to capture the convection as it moved through Memphis (below).

GOES-10 Visible Imagery (Click Image to play animation)

GOES-10 Visible Imagery (Click Image to play animation)

Tropical Storm Flossie approaching Hawaii

July 29th, 2013 |
Suomi NPP VIIRS 11.45 µm IR images

Suomi NPP VIIRS 11.45 µm IR images

A sequence of three Suomi NPP VIIRS 11.45 µm IR channel images (above) showed Tropical Storm Flossie (NHC | CPHC) as it was moving westward toward Hawaii during the 27 July – 28 July 2013 period. The first and third of the IR images were during daylight hours, while the second image was at night. The coldest cloud top IR brightness temperature on the final 28 July/22:32 UTC image was -80 C.

The corresponding Suomi NPP VIIRS 0.64 µm visible channel and 0.7 µm Day/Night Band images are shown below. Again, the first and third in the image sequence were during daylight hours; the second image in the sequence highlights the “visible imagery at night” capability of the VIIRS Day/Night Band, given sufficient illumination of clouds by Moonlight. On this particular day, the Moon was in the “Waning Gibbous” phase, at 55% of full — still providing ample illumination of the cloud features associated with TS Flossie at 11:18 UTC or 1:18 AM local time. A comparison of the 11:18 UTC 0.7 µm VIIRS Day/Night Band and 11.45 µm IR images can be seen here.

Suomi NPP VIIRS 0.64 µm visible channel and 0.7 µm Day/Night Band images

Suomi NPP VIIRS 0.64 µm visible channel and 0.7 µm Day/Night Band images

 

MIMIC Total Precipitable Water product with surface analyses

MIMIC Total Precipitable Water product with surface analyses

The MIMIC Total Precipitable Water product with overlays of surface analyses (above) showed that Flossie was tapping moisture from the Intertropical Convergence Zone (ITCZ) / Monsoon Trough, which was located around 10 N latitude. An animation of the MIMIC TPW product is shown below, covering the 27 July – 29 July time period.

MIMIC Total Precipitable Water product (click image to play animation)

MIMIC Total Precipitable Water product (click image to play animation)

A comparison of 0.7 µm VIIRS Day/Night Band and 11.45 µm IR channel images at 11:00 UTC (1:00 AM local time) on 29 July (below) revealed that the low-level circulation center was displaced far to the northeast of any remaining deep convection (which was confined to the southeast quadrant, but still exhibited cloud-top IR brightness temperatures as cold as -75 C). In addition, the Day/Night Band image was used by the Central Pacific Hurricane Center to re-adjust the position and resulting forecast track of Tropical Storm Flossie (CPHC forecast discussion), since the “visible image at night” showed that the center of the tropical cyclone was farther north than expected.

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

===== 30 July Update =====

A comparison of Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images at 12:12 UTC or 2:21 AM local time on 30 July (below) showed the weakening remnants of Tropical Storm Flossie (now a Tropical Depression) centered just north of the island of Kauaʻi.

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

 

Dorian

July 25th, 2013 |
GOES-13 (left) and GOES-12 (right) visible imagery

GOES-13 (left) and GOES-12 (right) visible imagery images

Tropical Storm Dorian has formed in the far eastern Tropical Atlantic, just west of the Cape Verde Islands. The visible imagery above, from GOES-13 (left) and GOES-12 (right) shows the evolution of the storm in the morning on July 25th. The position change in six hours between 0845 UTC and 1445 UTC suggests a steady west-northwest movement. GOES-12 (scheduled to be decommissioned on 16 August) sits above 60 W vs. 75 W for GOES-13. Accordingly, GOES-12 has a more top-down view of the storm (near 35 W on July 25th) and GOES-13’s view is more oblique. This explains the more circular presentation in the GOES-12 imagery compared to GOES-13. A similar difference in geometry is apparent in the 10.7 µm imagery shown below. A slow increase in the storm organization is also obvious in the animation below.

GOES-13 (left) and GOES-12 (right) 10.7 µm imagery

GOES-13 (left) and GOES-12 (right) 10.7 µm imagery imagery

MIMIC Total Precipitable Water, 1300 UTC 25 July 2013

MIMIC Total Precipitable Water, 1300 UTC 25 July 2013

Future development of the storm depends strongly on the environment through which the storm will move. Morphed microwave data (above) suggests that the atmosphere ahead of the storm is dryer, as total precipitable water values are less than 40 mm in the central tropical Atlantic. However, there is robust feed of moisture from the Equator near the South American coast into the storm and ocean surface temperatures are progressively warmer along the projected path of the storm. Sounder data from this site, shown below, suggests that Dorian is moving into a dryer and more stable environment. (In the imagery below, Dorian is the small storm at the far eastern edge of the sounder footprint, near 15 N and 35 W)

GOES-12 Sounder DPI values of Lifted Index and Total Precipitable Water, 1300 UTC on 25 July 2013

GOES-12 Sounder DPI values of Lifted Index and Total Precipitable Water, 1300 UTC on 25 July 2013

ASCAT Scatterometer winds, and Total Precipitable Water, around 1100 UTC on 25 July 2013

ASCAT Scatterometer winds, and Total Precipitable Water, around 1100 UTC on 25 July 2013

Scatterometer winds from ASCAT, above, show tropical storm-force winds north of the storm center. The storm itself is embedded within a rich moisture feed from the ITCZ. Further information and graphics on the storm are available at the CIMSS Tropical Weather Website and the CIMSS GOES-R Tropical Overshooting Tops Website. Refer to the National Hurricane Center website for official forecasts and storm discussions. People along the southeast coast of the United States and in the Caribbean Sea should monitor the progress of this early-season storm.